This invention relates to a method of forming large minimal grain or crystal copper blocks and particularly to the forming of an essentially single-grain or crystal copper in sizes in excess of one and one-half inches in diameter.
Various uses of copper require relatively high purity copper because of the effects of the release of trapped impurities, such as gases and the like, under operational conditions. Often, copper which is generally sold as an oxygen-free, high conductivity copper includes a substantial number of individual grains. The impurities in copper are generally trapped at the grain boundaries. Such impurities can be liberated under certain operating conditions, producing undesirable results. This can occur, for example, wherein such copper is included in the conductive components of power system vacuum interrupters, widely employed in the power industry, plasma devices and the like. This is particularly true of internal interrupter elements, such as, contacts, and the like, which are subject to arcing under operating conditions. Generally, the arcing of such contacts tends to erode the surface copper and liberates any impurities therein. Thus, great care is taken in the casting and processing of copper to form such contacts to minimize the presence of impurities, thereby increasing the performance characteristics of the interrupter.
Although current interruption is preferably made with pure copper content, normal reclosing under power on conditions could result in welding the contacts to each other such that the interrupter mechanism could not open the contacts on the next demand for interruption. Such welding is conveniently minimized by the addition of small amounts of antiwelding materials, such as, bismuth, to the copper from which the contact is formed. The additive material may, however, significantly degrade the interrupting characteristic of the contact. To overcome the latter, the contact is generally designed to locate the initial arc on a first portion of the contact surface and then move the arc across the contact surface to a second portion. This permits construction of a composite contact with the first portion including the antiweld material and the second portion of an essentially pure copper. Such contacts are generally formed with an inner button portion connected to an outer annular portion at the periphery or via a curved intermediate runner portion. The inner and outer portions are axially offset such that the arc runner portions of opposed contacts are more closely spaced and the arc is initiated thereat moving therefrom to the main, highly conductive portions.
Interrupter contacts of essentially pure copper having a diameter of approximately five inches, are generally employed in the power systems industry. The refined copper used in forming such contacts may be melted and recrystallized a number of times under special procedures to form contacts or blocks of a diameter for separation into a plurality of contact buttons for use in the interrupters. Generally, the copper is cast into an ingot having the contact diameter. It is then cut and machined into the appropriate contact for a particular vacuum-type interrupter. The finished contact is mounted within the vacuum chamber of the interrupter in a suitable manner to prevent contamination of the electrode or contact. Oxygen or other impurities trapped within the copper grain structure or matrix may be minimized during the casting process by the agitation of the liquid metal or by the addition of various gettering materials or agents. For example, U.S. Pat. No. 2,054,923, issued Sept. 22, 1936, suggests agitation of the melted metal to expose the metal to the surface within a vacuum. U.S. Pat. No. 1,948,316 suggests the addition of a gas reducing hydrogen to the melt under vacuum, while U.S. Pat. Nos. 3,738,827 and 3,776,719 suggest the addition of a gettering agent to the melt. Still other patents, such as U.S. Pat. No. 3,234,351 have suggested directional cooling during crystallization to develop a crystallizing multiple crystal copper ingot having grains whose surface to volume ratio is no greater than six per centimeter. However, relatively large contacts for vacuum interrupters would include a substantial number of crystals or grains requiring that special care be taken to minimize any trapped gaseous constituents.
Still another U.S. Pat. No. 3,470,936 discloses a method of forming high purity copper for the purpose of improving the cold working characteristics thereof as well as its resistance to corrosion. It can be seen that the need for a highly improved high purity copper, and particularly one with a large essentially single-grain or single-crystal structure in order to minimize entrapment of impurities at the grain boundaries, exists.